This invention relates to a method of bonding a window to a substrate using an adhesive without the need for a primer.
Polyurethane sealant compositions typically are used for bonding non-porous substrates, such as glass, to nonporous substrates, these are described in U.S. Pat. No. 4,374,237 and U.S. Pat. No. 4,687,533, both incorporated herein by reference. U.S. Pat. No. 4,374,237 describes a polyurethane sealant containing urethane prepolymers which have been further reacted with secondary amine compounds containing two silane groups. U.S. Pat. No. 4,687,533 describes a polyurethane sealant containing urethane prepolymers which contain silane groups which have been prepared by reacting a polyisocyanate having at least three isocyanate groups with less than an equivalent amount of an alkoxysilane having a terminal group containing active hydrogen atoms reactive with isocyanate groups to form an isocyanatosilane having at least two unreacted isocyanate groups. In a second step, the isocyanatosilane is mixed with additional polyisocyanate and the mixture is reacted with a polyol to form a polyurethane prepolymer having terminal isocyanato groups and pendant alkoxysilane groups.
However, when such sealants are used to bond glass substrates to painted substrates, such as for window installation in vehicle manufacturing, the lap shear strength of the bonded substrate may be less than desirable for safety or structural purposes. Consequently, a separate paint primer comprising a solution of one or more silanes or isocyanates is typically applied to a painted substrate prior to the application of the sealant in most vehicle assembly operations for bonding the windshield and the rear window. The use of a primer in assembly operations is undesirable in that it introduces an extra step, additional cost, the risk of marring the paint surface if dripped on an undesired location, and exposes the assembly line operators to additional chemicals.
Automotive OEMs have demanded that the paint suppliers provide more robust acid resistant paint systems which stand up to harsh environmental conditions. These paint systems are difficult to bond to due to the increased chemical resistance of the paints. One problem with developing an adhesive which bonds to these paint systems on an automobile is that there are several different paint chemistries. Examples of paints which are difficult to bond to are the DuPont silanated urethane melamine, the PPG carbamate melamine paints and the acid resistant acrylic melamine paints from DuPont. What is needed is a method of improving the adhesion of adhesives to painted substrates.